mailbox/omap: remove the private mailbox structure

The structure omap_mbox_priv is used previously to store arch
specific (OMAP1 vs OMAP2+) data, and is no longer required to be
maintained separately. Instead, absorb its elements into either
the sub-mailbox device structure, omap_mbox, or the individual
fifo descriptor structure, omap_mbox_fifo.

The newmsg_bit and notfull_bit used on Rx and Tx fifos respectively
are represented by the new intr_bit field in the fifo descriptor
structure. The interrupt configuration registers are also moved
into the fifo descriptor structure to allow the Rx and Tx fifos
to use different interrupt lines/users.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>